Seismic interpretation is the science and art of deducing geologic history by delineating geological surfaces a represented in and by seismic data. These surfaces represent boundaries between layers of rock in the Earth, and knowing the geometry of the layers is important for understanding the history of the rocks. Layer geometry may reveal the presence of buried river channels or beaches. If layers are tilted, bent, or broken, the geometry may reveal that rocks moved after they were buried. Seismic data provide a relatively inexpensive way to discover subsurface geometry.
Seismic data are typically produced by transmitting acoustic signals (generated by dynamite, for example) into the Earth and recording the echoes. The Earth typically consists of rocks deposited as layers, and the acoustic properties of rock typically change from layer to layer. Changes in acoustic properties cause echoes at layer boundaries, and these boundaries constitute “surfaces of reflection.” Subsurface geometry can be deduced, in varying degrees, by study of these surfaces. Interpreters, e.g. users or computer programs or both, want to know the position, shape, and orientation of subsurface layers and whether they are broken or continuous.
Rocks in the subsurface are generally porous, similar to beach sand but with less ability to absorb fluids. Such rocks contain either water, oil, or gas. Oil and gas are lighter than water and float upward in the subsurface as they do at the surface. The path of movement and the cessation of movement is in large part dependent on the geometry of the subsurface layers in which the fluids move, and this makes the geometry of interest to seismic interpreters.
The geometry also allows the interpreter to deduce how the rocks came to be deposited, whether at the mouth of a delta, whether upstream as sand bars or mud in overbank flooding, or whether the rocks were deposited offshore from a delta in deeper water. This history can be important in more sophisticated analyses regarding the generation of oil and gas and the distribution of pores in the subsurface.
FIG. 1 depicts a typical seismic echo as detected by a single receiver at the surface. It is a sinusoidal curve as a function of time. The strength of the echo oscillates between compression and rarefaction over a period of several seconds, and this rise and fall in pressure with time is recorded for processing and analysis. A single recorded echo is called a “seismic trace.” FIG. 2 shows the same trace enhanced for interpretation purposes. Where the signal rises above zero (into compression), the line marking the signal has been filled in with black. The configuration of a trace reflecting to the right and then returning toward zero, as highlighted in black, is called a “peak” in the trace.
To detect geometric relationships, echoes must be collected along a line or over an area, so seismic receivers are typically laid out along a line or in a grid pattern over the surface of the Earth. Capture over an area permits comparison of echoes from location to location. Each receiver, in effect, affords a “peephole” into the subsurface, and geometry is detected, e.g. viewed, from many such adjacent peepholes.
Even when collected in grids, seismic data are typically interpreted as lines, or vertical “sections” of data, each line or section being only part of a grid when data are collected in a grid pattern. A line display provides a profile view of the seismic echoes so that one can readily see differences in the chain of echoes along the line.
FIG. 3 depicts a typical seismic line as a collection of traces such as shown in FIG. 1, and FIG. 4 shows the same line as typically displayed for interpretation, using the display technique of FIG. 2.
One aspect of interpretation is the mechanical marking of surfaces deemed important by the interpreter. Marking these surfaces is done by interpreters when they draw lines on the seismic section. Each line represents the presence of an interpreted surface at that location. An interpretation project will typically generate several dozen and sometimes hundreds of surfaces. Each surface is represented by lines drawn on numerous seismic sections. Surfaces are typically called “horizons,” and the surface name is the “horizon name.” Each surface is typically named distinctively, in part so that the interpreter can recognize something of its importance to geological understanding.
Horizons may be displayed in color so that they can be distinguished from one another and from the seismic data itself. FIG. 5 is the same as FIG. 4 with a typical seismic interpretation added. This is a simple interpretation in which the interpreter has followed seismic peak amplitudes across parts of the section.
One geometry of interest for the present invention is the geometry of repeated surfaces. FIG. 6 is a duplicate of FIG. 5 with additional interpretation. The second interpretation has the same color as the first, and it is intended to represent the same surface but at a different depth. This type of surface is sometimes called a “repeated” or “repeating” surface. This surface in FIG. 5 is also present in two separated locations, but the locations are separated laterally. “Repeated” is reserved for multiple occurrences of a surface vertically. For example, if a well were drilled at the location indicated in FIG. 6 the same surface would be encountered twice by the well. This would be detected by a repeat of rock samples taken from the well.
Layers of rock may have been physically “pushed,” e.g. from left to right. Ramp-like surfaces of slippage over which these huge pieces of rock once slid may be indicated, e.g. by using color, and may further be annotated, e.g. with small arrows indicating movement of the rock above over the rock below the surface. Such surfaces are known as “thrust faults”, and the layers of rocks are known as “thrust sheets.” Thrust faults, and hence repeated surfaces, are common in some hydrocarbon-bearing provinces such as western Colorado, western Canada, and western Colombia. Vendors of seismic interpretation software are well aware of the need for a means to interpret repeated surfaces in part because requests for this ability are frequent.
Representation of repeated surfaces is a challenge when surfaces are represented on a computer, as in FIG. 6. When computers are used for seismic interpretation, the interpreter “draws” the surface on the computer screen, e.g. with the mouse cursor. This is very like tracing the surface with a pencil, and the computer mimics the pencil by displaying the path of the cursor as colored lines on the seismic image, such as are shown in FIGS. 5 and 6.
To correct a horizon, the interpreter redraws the line, and the old line is automatically erased where it overlaps the existing line. FIG. 7 shows a first step that may be present when revising the interpretation shown in FIG. 6. Re-interpretation began at point A and is presently at point B.
The computer recognizes that the new line is at the same position on a map but at a different vertical position and erases the original interpretation, higher or lower, as the new interpretation is added. This convenience works very well unless the surface is a repeated surface and the new interpretation is intended to indicate repetition of a surface, not a correction to a surface.
The horizon name may be key to whether a new interpretation is added or an existing interpretation is replaced. For example, if the name is the same as an existing horizon at the given location, the new drawing may be recognized as a correction and the existing marking may be erased where it overlaps the new marking. If the name is different, the new line may not erase the existing line.
FIG. 8 illustrates the ambiguity of reinterpretation. If the two horizons are in fact the same, where such cannot be distinguished by the computer, the same procedure that generated FIG. 7 could also generate FIG. 8.
A user could instruct the computer to add new lines regardless of existing lines, but this would eliminate the convenience of automatic correction, and correction is a very common process. If old lines were not removed automatically, interpretation would be a much more time-consuming, and therefore costly, process because interpreters would have to erase as much as they reinterpret. Interpreters may be well acquainted with the difficulty of interpreting a repeating surface, but typically none propose that removing automatic erasure is a workable solution.
For purposes of presentation, interpreters can assign the same identifier, e.g. color, to all horizons that represent a given repeated surface. A reviewer of the data will then see them as the same surface repeated. Internally, the horizons have different names so they can overlap without the implied erasure problem. The interpreter may assign similar names so that horizons can be recognized as related in a list. “Eocene 1” might represent one component of the “Eocene” surface while “Eocene 2” represented another component.
Vendors of software for seismic interpretation often receive a request for improving the handling of repeated surfaces. One focus in this effort has been to find a way to draw a line using an existing name while appropriately erasing or not erasing other lines with the same name. A key to interpreting a repeated surface is development of a method that allows users, e.g. interpreters, to manage, not draw, a repeated surface as one surface. Most of the effort toward solving the “repeated surface problem” has been focused on how interpreters mark the surface on the computer screen, but this is a small part of the problem. If the problem were solved in the marking methodology, then the parts would have a common genesis that would facilitate common management, but a common genesis is not essential to meeting the need for repeated surfaces, whereas common management is essential.
Repeated surfaces are typically created and manipulated as single surfaces. The result is that some operations with repeated surfaces are either not possible or are very inconvenient.